1. Field of the Invention
The present invention relates to organometallic polymers. More particularly, the invention relates to optically transparent polyvalent metal-containing monomers and polymers and processes for preparing them.
2. Description of the Prior Art
Organometallic polymers have been prepared primarily from ethylenically-based monomers. For example, U.S. Pat. No. 3,974,104 discloses that a tin-containing copolymer can be obtained by copolymerizing an ester of trialkyl tin and an ethylenically unsaturated acid (acrylic acid, etc.) with one or more ethylenically unsaturated monomers. The copolymers thus formed contain about 2.5-10% by weight of tin and are noted to be advantageously used as radiopaque and transparent denture base materials. Similar copolymers have been formed from, for example, lead dimethacrylate and lead dimethylmethacrylate. Transparency is lost however at only 6% lead methacrylate content.
A variety of methods have been devised to increase the metal content of these polymers in order to enhance their ability to act as radiation barriers while at the same time still maintain their optical transparency. Such efforts have included adding radiopaque materials to polymeric powders and to monomeric liquids in simple admixture or in the polymerization procedure. The admixture approach has, however, not attained the success desired principally because such techniques greatly decrease the physical properties of the final product. Further, many of the radiopaque materials migrate (bleed) from the polymer substrate.
One of the most successful approaches for producing radiopaque, transparent polymers of high metal content has been by using a combination of the above-two general methods. For example, U.S. Pat. No. 4,129,524 discloses a radiation shielding polymer composition that comprises
(a) a copolymer of an alkyl methacrylate, a hydroxyalkyl acrylate, a hydroxyalkyl methacrylate or styrene and
(b) lead diacrylate or lead dimethacrylate in combination with a lead carboxylate. The ratios of component (b) to lead carboxylate are critical to the preparation of a sufficiently transparent, radiopaque polymer having good mechanical strength. Accordingly, the maximum lead content in the composition is about 32% by weight, corresponding to a lead equivalent of about 1.1 mm lead per inch of radiation shield. Effective radiation barriers are difficult to fabricate from these compositions because of the thickness necessary to be an effective barrier and once formed are undesirably heavy.
High metals-content, transparent organometallic polymers have been heretofore unavailable.
It is an object of the present invention to provide a transparent organometallic polymer of higher metals content without the use of additives.
It is a further object of the present invention to provide a novel transparent radiation shielding material.
It is an additional object of the present invention to provide a process for producing a transparent radiation shielding organometallic polymer of high metal content without the use of additives.
These and other objects can be attained by reference to the Detailed Description of the Preferred Embodiment.